This invention is directed to a cathodic electrocoating composition and in particular to a clear non-pigmented cathodic electrocoating composition.
The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent usually a blocked polyisocyanate and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with pigment, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions, resin compositions, coating baths and cathodic electrodeposition processes are disclosed in Jarabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
There currently is a demand for electrocoating compositions that are clear and non pigmented. Such compositions have greater stability in the electrocoating bath since there are no pigments that can settle out. Improved bath stability allows the bath to be operated under higher temperatures and cooling is either not needed or significantly less cooling is needed which reduces energy cost for cooling the bath. Electrocoating composition that do not contain pigments are much easier to manufacture since pigment dispersions are not required to be separately formed as is done with conventional pigment. Pigment dispersions are relatively difficult to manufacture and are expensive. By eliminating the use of pigment dispersion, results in substantial savings in time and materials. Also, coverage per unit of coating composition is increased due to the removal of high density pigments.
There is a problem with formulating such a clear coating composition. Alkyl tin oxide catalysts such as dibutyl tin oxide are used in the composition to catalyze the reaction between the polyepoxide resin and the blocked isocyanate crosslinking agent. This catalyst is a solid that is not water soluble and conventionally is dispersed in water often with pigments and then added to the electrocoating composition. Having the catalyst dispersed in the electrocoating composition would not provide a clear composition when it is electrodeposited and the catalyst can settle out of the electrocoating bath. Both are undesirable. It would be desirable to formulate a clear coating composition having a water soluble catalyst.